Japanese Unexamined Patent Application Publication No. 2001-85748 (hereinafter referred to as patent document 1) and Japanese Unexamined Patent Application Publication No. 2001-148514 (hereinafter referred to as patent document 2) propose a light emitting device which comprises a LED chip, a circuit board mounting the LED chip, a metal frame (e.g. made of aluminum) surrounding the LED chip on a surface of the circuit board, and an encapsulation member (e.g. made of an transparent resin such as an epoxy resin and a silicone resin) filled within the frame to encapsulate the LED chip and bonding wires connected to the LED chip. The frame disclosed in the Patent document 1 and 2 is shaped to have an opening area which becomes greater as it is spaced further away from the circuit board and is finished to have a mirror interior face which serves as a reflector reflecting a light emitted from the LED chip.
Further, the patent document 2 discloses the light emitting device which utilizes a blue LED chip with blue luminescence to radiate a white luminescent spectrum by use of a yellowish fluorescent material which is dispersed in a transparent resin encapsulating the blue LED chip to be excited by the light from the blue LED chip.
When an epoxy resin is used as a material of the encapsulation member in the light emitting device, bonding wires may be cut off when subjected to a heat cycle test (temperature cycle test) of repeating a low temperature period of −40° C. and a high temperature period of 80° C. alternately, due to thermal-expansion of an electrically conductive pattern mounted on a substrate of a circuit board in the high temperature environment.
On the other hand, when a silicone resin is used as a material of the encapsulation member in the light emitting device, the bonding wires can be prevented from being cut off in the high temperature period of the heat cycle test because of that the encapsulation member is gelatinous and elastic. However, since a linear expansion coefficient of the silicone resin, which is a material of the encapsulation member, is more than 10 times that of an aluminum which is a material of the frame, voids may be generated within the encapsulation member due to the difference in linear expansion coefficient between silicone resin and aluminum.
Furthermore, in the light emitting device, although the light emitted from the LED chip can be taken to external of the encapsulation member effectively by means of the frame finished with a mirror interior face, there is a failure of causing light loss resulting from the reflection of the light on the interior face of the frame.
Besides, in the light emitting device according to the Patent document 1, when a lens controlling an orientation of the light emitted from the LED chip is arranged over the encapsulation member and the frame, a light output may be lowered due to a deviation between an optical axis of the LED chip and that of the lens, resulting from dimensional accuracy or assembling accuracy of the frame and the lens.
The Patent document 2 also discloses a light emitting device in which the encapsulating member of the LED chip and the bonding wires connected thereto are configured to have its portion shaped into a convex lens. However, since the light emitting device utilizes a fluorescent material which is dispersed partially or entirely in the encapsulation member, its concentration is likely to vary from portions to portions in the encapsulation member, causing color ununiformity. Further, when the fluorescent material is dispersed entirely in the encapsulation member, an increased amount of the fluorescent material is necessary with an attendant cost increase. It is possible that a dome-shaped color conversion member is molded from a fluorescent material and a transparent resin material and is arranged to form an air layer around the encapsulation member. However, in this instance, the color conversion member has to be relatively large in size, and requires an increased amount of the fluorescent material. Further, the air layer between the conversion member and the encapsulation member may cause a total reflection at the interface between the encapsulation member and the air layer depending upon the shape of the encapsulation member, thereby reducing the light output.